It is common prior art practice to seal a shaft with a packing arrangement formed by a plurality of stacked V-shaped rings. The V-shaped rings, commonly known as chevron rings, are generally disposed in a cylindrical packing chamber of a structural body in circumferential sealing relationship to the shaft. Follower rings are typically used to apply axially pressure against the stacked rings, and to axially compress the V-shaped rings in the packing chamber. Such axial compression tends to radially expand the rings and to assist in maintaining a sealing relationship between the stacked rings and the shaft (on the radially inward side of the V-shaped rings) and between the stacked rings and the cylindrical sidewall of the packing chamber (on the radially outward side of the rings).
Such V-shaped rings have been formed from a wide range of materials in the past. It is common, for example, to use elastomeric V-shaped rings formed from homogeneous rubbers in applications where relatively low pressures are encountered and where the fluid media does not damage the rubber. Similarly, fabric V-shaped rings coated with elastomers are often used on heavy duty equipment or when higher pressures are encountered. However, rubber and fabric materials are frequently unacceptable when corrosive media is handled since the corrosive material will often attack both the rubber and the fabric material. Under such conditions, the materials of choice for such V-shaped rings are molded fluorinated hydrocarbon polymers. One such fluorinated hydrocarbon polymer in common use is polytetrafluoroethylene. Many of these fluorinated hydrocarbons, including polytetrafluoroethylene, are inert to virtually all chemical media and are suitable for use with a wide variety of corrosive fluids. In addition, many of the fluorinated hydrocarbon polymers have extremely low coefficient of friction.
A problem arises, however, from the fact that valves used to control the flow of corrosive process fluids almost always are formed of metals, and that fluorinated hydrocarbon polymers have a high coefficient of expansion relative to most metals. In some industrial situation, valves are thermally cycled through extreme temperature ranges, as for example between very high temperatures of several hundred degrees F. to very low temperatures of 40 degrees below zero or lower. When cooled after exposure to elevated temperatures, fluorinated hydrocarbon polymers shrink more than the metallic components of a valve with which they are in contact. As a result, the sealing effectiveness between the radially outward sides of these fluorinated hydrocarbon polymers and the sidewall of the packing chamber is significantly reduced when the valve is cooled, even when the packing rings are initially compressed very tightly.
There have been many attempts to overcome the problems resulting from thermally induced relative dimensional changes between the metal valves and sealing material formed from fluorinated hydrocarbon polymers. For example, in U.S. Pat. No. 4,475,712 to DeJager, a patent assigned to the assignee of the present invention, a valve designed for operation under severe conditions, including extreme temperature ranges is disclosed. The valve disclosed in this patent houses the actuating shaft in a portion of the valve body that is integral to the valve portion defining the internal flow passage for the process fluid being controlled by the valve. The valve shaft is sealed by a sealing assembly that includes an annular seal ring and a two sets of polymeric V-shaped packing rings. The DeJager valve body includes an annular projection located at the outboard end of the bore (distal to the valving member) that extends inwardly (toward the valving member) into the bore, and in circumferentially spaced relationship to the shaft. The annular seal ring is fitted into the space between the projection and the cylindrical sidewall of the bore. The two stacks of V-shaped packing rings are disposed interiorly of the annular seal ring, between the shaft and cylindrical sidewall. The sealing assembly also includes a lantern ring located about the shaft between the two sets of V-shaped rings.
Like most valves for corrosive fluid media, the DeJager sealing assembly is formed of a material having a coefficient of thermal expansion which is substantially greater than the coefficient of expansion for either the valve body or the valve shaft. Consequently, when the valve is cooled to low temperatures, the sealing assembly tends to pull away from the cylindrical sidewall of the bore, thereby reducing the sealing effectiveness between the sealing assembly and the cylindrical sidewall. In order to protect against leakage of the process fluid media, the DeJager valve relies upon the seal between the inwardly extending annular projection and the annular seal ring.
While the sealing assembly for the shaft of the valve disclosed in the Dejager patent is very effective for stopping leakage of the process media along the valve shaft, even under conditions of thermally cycling through extreme temperature ranges, it suffers from the disadvantage of permitting considerable leakage in the space between the outer radial surfaces of the packing rings and the cylindrical sidewalls of the packing chamber whenever the valve is subjected to extremely low temperatures. Among other disadvantages, such leakage permits the process media to travel for a substantial distance from the process line, past the lantern ring interposed between the sets of packing rings, and close to the ambient atmosphere. Since the sealing assembly of the DeJager valve permits leakage past the lantern gland, it is not fully suitable for leak sensing at that location. Moreover, it is not fully suitable for the application of an inert fluid at the lantern ring at a pressure that is in excess of the process fluid pressure, as disclosed in U.S. Pat. No. 4,531,537, also assigned to the assignee of the present invention. In such pressurized inert fluid applications, any failure of the seals allows the flow of the higher pressure inert fluid into the process line, and does not allow the process fluid escape the confines of the valve body.